Aircraft landing and takeoff operations are particularly dangerous in adverse weather or under conditions of limited visibility. Aircraft, particularly helicopters, are very difficult to operate during landing when the pilot is unable to observe adequately the motion of the helicopter with respect to the landing area. For example, as a helicopter approaches a landing site in a desert, downwash from the rotors typically stirs up the sand, which greatly reduces visibility. This is very dangerous because the pilot cannot adequately observe the landing area to obtain the necessary visual feedback of the aircraft's speed and position. A helicopter is particularly vulnerable if the pilot is unaware that the helicopter is drifting laterally as it touches down. Similarly, lateral drift resulting from an unexpected cross wind may cause the aircraft to roll, as for example when a skid engages the ground during takeoff or landing.
While optical systems for measuring air speed are known, such as that shown in U.S. Pat. No. 5,272,513, known altitude and ground speed detection systems are too slow when used in conditions of low visibility to allow the pilot, or even automatic equipment, to respond quickly enough for the aircraft to be maneuvered safely when near the ground.
Further, in a military context those systems that use radio frequency (RF) emissions to measure the altitude of an aircraft during landing, for example, can be detected and reveal the location of the aircraft to the enemy.